Electrophotographic imaging processes have been described extensively in patents and other literature. These processes have in common the forming of an electrostatic charge pattern on an insulating photoconductor. The pattern, or latent electrostatic image, is made visible by contact with a developer containing electrostatically charged toner powder. Several methods of dry development are available, including the well known magnetic brush and cascade development methods. Many dry developers are a mixture of toner particles and carrier particles. For magnetic brush development, the latter can be a magnetic substance such as iron filings, iron powder or iron oxide. For cascade development and other methods the carrier particles can be non-magnetic substances such as glass or ceramic beads. The toner particles become triboelectrically charged by frictional contact with the carrier particles. Then, when contacted with the oppositely charged image pattern on the photoconductor, they adhere to the charged areas and make the image visible. In well-known office copying machines, the developed toner image is transferred from the photoconductor to a receiver sheet, e.g., plain paper, to which it is fixed by fusion or other known techniques.
While polymeric binder is a major component of the toner, certain addenda usually are dispersed in the polymer. These can include one or more colorants such as pigments and dyestuffs which make the developed charge pattern visible. Also desirable as addenda are compounds which help to maintain a uniform, stable, high net electrical charge on the triboelectrically charged toner particles. These compounds are known as charge-control agents.
A variety of charge-control agents for dry toner particles have been proposed. For example Jadwin U.S. Pat. No. 4,394,430 issued July 19, 1983 (which is herein incorporated by reference in its entirety) describes an electrostatic toner containing alkyldimethylbenzylammonium salts of the formula ##STR1## wherein R is an alkyl group of from 12 to 24 carbon atoms and X.sup.- is an anion. These quaternary ammonium salts provide high uniform net electrical charge to a toner powder without reducing the adhesion of the toner to paper. In addition, they are versatile in their utility and toner compositions into which they are incorporated form good to excellent images. Furthermore, the fact that this type of compound is so mild that it has been used in skin lotions and therefore presents substantially no risk of skin irritation provides another advantage.
However, the quaternary ammonium salts disclosed in U.S. Pat. No. 4,394,430 have been found to be thermally labile, producing reactive by-products such as acids, olefins, amines, and organic halides when heated. These by-products may contribute to deleterious side effects as well as adversely affecting the triboelectrical behavior of the toner particles. For instance, by-products such as benzyl chloride, methyl chloride and amine hydrochloride are not effective charge-control agents. Therefore, degradation of the original charge-control agent into such reactive by-products reduces the effectiveness of the charge-control agent. The problem is especially vexing because many prior art quaternary ammonium salts are also very sensitive to their environment in the toner (e.g., humidity, chemical species, etc.) and to compounding conditions. As a result, it is difficult to control the extent of degradation, which leads to unpredictable changes in charge-control effectiveness.